Grabau — Biserial Arm in Certain Crinoids. 



291 



ditioDS found. This plate is completely wedge-shaped or 

 cuneate and extends entirely across from side to side. This 

 cuneate condition continues to the 87th plate, after which the 

 wedge becomes more and more truncated, and the upper and 

 lower sides more and more parallel, until in the 96th plate the 

 rectangular outline of the primitive plate is all but retained. 

 Thus there are at least twelve uni serial plates at the tip of this 



Fig. 1, a-h. 

 apical plates. 



Encrinns liliiformis. Diagram of arms showing uniserial 

 (Harv. Univ. Pal. Lab. Coll.) 



arm. The faint development of a tubercle on the 96th plate 

 suggests that this is not the terminal one, and that the arm is 

 therefore not quite perfect. (Compare fig. 2.) 



In an adjoining arm of the same individual (fig. lb) biserial 

 conditions continue to about the 80th plate, but there is a 

 slight inequality of development in plates YT and 78, the former 

 of which is developed at the expense of the latter. Plate 81 is 



